Heat exchangers such as the steam generators of a pressurized water nuclear reactor incorporate a tube bundle or nest constittuted by a very large number of small diameter tubes positioned vertically within a generally cylindrical bundle or nest envelope located within the external and very thick, rigid pressure envelope of the steam generator.
The pressurized water of the nuclear reactor flows within the small diameter tubes and the water to be vaporized is introduced into the bundle envelope, where it comes into contact with the outer surface of the tubes. The steam is recovered in the upper part of the bundle envelope and is then generally dewatered in steam-water separators located in the outer envelope of the steam generator above the bundle envelope.
Steam generators are very high units, in which the different coaxial envelopes are arranged with a generally small radial spacing.
It is known to use spacing plates into which pass the tubes of the bundle, and which are distributed in accordance with the bundle height, so as to keep the tubes in fixed, reciprocal radial positions. These spacer plates are interconnected by vertical tie rods, the assembly being placed within the bundle envelope.
It is therefore important to maintain the different coaxial envelopes of the steam generator and the assembly of the bundle by spacer plates in the radial directions, so as to avoid relative displacements and shocks between the the envelopes and the bundle in the case of external stresses, such as those accompanying an earthquake.
This radial maintaining or holding action is obtained by abutments fixed to the envelope of the bundle level with the spacer plates at a rate of several abutments for each spacer plate. These abutments are then regularly distributed on the periphery of the spacer plates.
Such a holding or securing device is known from FR-A-2,511,491. It is constituted by parts which are thicker than the bundle envelope and which have tapped holes within which it is possible to screw threaded studs, whose end projecting into the space between the bundle envelope and the rigid outer envelope bears against the inner surface of the outer envelope. The bearing action of these abutments is brought about after regulating the studs, by introducing wedge-shaped shims between the outer edge of the spacer plates and the inner surface of the bearing device directed towards the bundle.
Such abutments suffer from the disadvantage of occupying a considerable part of the width of the space reserved between the bundle envelope and the outer envelope and therefore significantly limiting the passage cross-section for the water to be vaporized flowing in the space and still without providing a perfect seal. These abutments also suffer from the disadvantage of requiring the introduction of shims into notches machined on the peripheral part of the spacer plates. These notches correspondingly decrease the distance between the peripheral tubes of the bundle and the edge of the spacer plates. If an adequate distance is to be maintained between the peripheral tubes and the plate, it is then necessary to reduce the number of tubes in the bundle. Moreover, abutments must have shapes which do not facilitate their fixing by welding in the bundle envelope wall.
Other devices for radially holding the bundle envelope and spacer plates of a steam generator are disclosed in FR-A-2,562,996 and U.S. Pat. Nos. 4,503,903, 4,583,584, 4,690,206 and 4,768,582. However, none of these devices are completely satisfactory. Their installation is generally complicated as a result of the use of shims for some of them. Certain of the devices do not offer an adequate bearing surface for holding the spacer plates.